Viruses are widely distributed throughout the Kingdom Fungi, including fungi pathogenic to animals and plants. Fungal infections continue to have a significant impact on human health and well-being. Similar to viruses of animals and plants, mycoviruses have potential utility for elucidating host biological processes and manipulating host phenotype. Progress in developing this potential is best illustrated by recent studies with members of the RNA virus family Hypoviridae. Hypoviruses persistently alter phenotypic traits, modulate gene expression and attenuate virulence of their fungal hosts. They are also the only symptomatic mycoviruses for which a reverse genetics system has been developed. Consequently, hypoviruses provide utility as biological control agents and as unique tools for identifying fungal virulence determinants and designing antimycotic therapeutic strategies. Substantial progress has been made in mapping hypovirus-encoded modifiers of cellular signaling pathways and in the characterization of previously mapped viral symptom determinants. Experimental approaches for unraveling hypovirus polyprotein processing pathways have been validated. The development of a host cDNA microarray platform has allowed the monitoring of global transcriptional responses to hypovirus infection. A hypovirus protein was shown to function as a suppressor of RNA silencing in a well characterized heterologous system. Three new Specific Aims have been designed to capture the full advantage of these recent advances and newly developed experimental approaches for further understanding and exploitation of hypovirus molecular biology while meeting the criteria of feasibility, relevance to ongoing studies and general significance: 1) further elucidate the ORFB polyprotein processing pathway, 2) investigate hypovirus-mediated reprogramming of host gene expression by comparing global cellular transcriptional responses to hypovirus infection and to modulation of G-protein/ cAMP signaling and 3) test the fundamental hypothesis that RNA silencing in fungi evolved as an antiviral defense mechanism. The results derived from completion of these coordinated specific aims will have immediate relevance to ongoing efforts to further enhance the utility of hypoviruses and other potentially efficacious RNA viruses for purposes of manipulating their hosts in novel and productive ways.